Foveal image inverter

ABSTRACT

An image-conducting optical fiber bundle extends along a central bundle axis between image input and image output ends. The bundle is twisted along a portion of its length such that an image inputted into the image input end is angularly displaced about the central bundle axis before being outputted through the image output end. Each constituent optical fiber includes a cladding with a cladding diameter corresponding with the fiber diameter of that fiber and a core with a core diameter. The ratio of the core diameter to the cladding diameter defines a core-to-clad diameter ratio relative to each fiber. In various embodiments, at least one of fiber diameter and core-to-clad diameter ratio varies as a function of a fiber&#39;s radial displacement from the central bundle axis.

PROVISIONAL PRIORITY CLAIM

Priority based on two previous provisional applications is claimed. Thefirst is Provisional Application Ser. No. 62/281,168 filed Jan. 20, 2016under the title FOVEAL IMAGE INVERTER, while the second is ProvisionalApplication Ser. No. 62/441,491 filed Jan. 2, 2017, also filed under thetitle FOVEAL IMAGE INVERTER. Moreover, the entirety of the previousprovisional applications, including the drawings, is incorporated hereinby reference as if set forth fully in the present application.

BACKGROUND

The transmission of images and, more generally, electromagnetic waves,through bundles comprised of adjacently fused optical fibers is anestablished art. Image conduits such as inverters (i.e., imageinverters), tapers and “straight-throughs” are well known topractitioners of the fiber optic arts. Fused optical fiber imageconduits find broad application as components in such devices as nightvisions goggles, rifle scopes, x-ray detectors and medical imagingapparatuses, by way of non-limiting example.

The basic fabrication techniques of each of the examples listed abovehave process steps in common. For instance, the most basic of fusedoptical fiber image conduits is a one-to-one, linear conduit having aninput (e.g., image receiving) end and an output (e.g., image emitting)end. Light reflected from an object adjacent the input end enters theinput end as an image. The image is conducted through the conduit andexits the output end from which a detector device or human eye sensesit. In a simple one-to-one conduit, the image exits the output endwithout intentional alteration. For instance, the image is notmagnified, reduced or angularly displaced about the longitudinal axis ofthe conduit.

Referring to FIGS. 1A and 1B, as is known in the art, a basic one-to-oneimage conduit (FIG. 1A) is an intermediate product in the fabrication ofan inverter (FIG. 1B). To fabricate an inverter, a one-to-one conduit isheated to an appropriate temperature. One end of the conduit in thenangularly displaced (i.e., twisted) about the longitudinal axis of theconduit with respect to the opposite end. In the case of an inverter,the one end is twisted 180 degrees with respect to the other end. Whenproperly controlled and executed, this process produces an inverter inwhich the original configuration of the face at each of the input andoutput ends is maintained, but in which one end is inverted with respectto the other. Accordingly, an image entering the image-receiving end isrotated as it is conducted through the constituent fibers within thefused bundle and exits the image-emitting end inverted.

It will be readily appreciated that, as the heated bundle is twisted toangularly displace about the longitudinal axis of the conduit one endwith respect to the opposite end, constituent fibers within the bundleare stretched lengthwise. Moreover, fibers that are more toward theperiphery of the bundle are stretched to a greater extent than fibersthat are more centrally located. As a result, fibers more toward theperiphery decrease in diameter more dramatically than do fibers moretoward the center, particularly in central regions along their lengths.Because, according to traditional fabrication methods, the constituentfibers of the bundle are all of the same cross-sectional dimensions,peripheral fibers are sometimes stretched and constricted to such anextent that their ability to transmit light efficiently is negativelyimpacted, which results in undesired image effects, includingvignetting. One way of avoiding image-degrading constriction ofperipheral fibers is to twist the bundle the desired number of angulardegrees over a longer bundle length. However, this results in bundlesthat may be too long, heavy and unwieldy for use in the intendedenvironment or application.

Accordingly, a need exists for a fused fiber bundle and method offorming an inverter from the same that facilitates image inversion overa relatively short bundle length while obviating the undesiredperipheral image degradation associated with previous fused-bundle imageinverters.

SUMMARY

An illustrative embodiment of an imaging-conducting optical fiber bundleextends longitudinally along a central bundle axis between an imageinput end and an image output end. Between the opposed image input andoutput ends there extends a plurality of mutually and adjacently fusedconstituent optical fibers. Each constituent optical fiber has a firstend coinciding with the image input end and a second end coinciding withthe image output end. In various embodiments, each constituent opticalfiber is configured as an imaging fiber capable of conveying aninfinitesimal portion of an input image from the between the image inputand image output ends. Moreover, the optical fiber bundle is twistedabout the central bundle axis and along a portion of its length suchthat an image inputted into the image input end is angularly displacedabout the central bundle axis before being outputted through the imageoutput end.

In keeping with general convention, each constituent optical fiberincludes an optically transmissive core about which there is collapsedand fused an optical cladding, the core and cladding having relativeindices of refractive that facilitate propagation of electromagneticwaves through the core by means of total internal reflection. Thecladding is defined by a cladding diameter that corresponds with thefiber diameter, while the core is defined by a core diameter. The ratioof the core diameter to the cladding diameter defines a core-to-claddiameter ratio relative to each fiber.

In some versions, the fiber diameters of the constituent optical fibersincrease as a function of radial displacement from the central bundleaxis. That is, as viewed into a selected cross-section of the bundletaken orthogonally to the central bundle axis, optical fibers moretoward the outer boundary of the bundle have larger fiber diameters thando optical fibers more toward the central bundle axis.

In each of some alternative embodiments, the core-to-clad diameterratios of the constituent optical fibers vary as a function of radialdisplacement from the central bundle axis. That is, optical fibersnearer the central bundle axis exhibit core-to-clad diameter ratiosdisparate by design from the core-to-clad diameter ratios of opticalfibers situated more radially distant from the central bundle axis. Inat least one version, as viewed into a cross-section of the bundle takenorthogonally to the central bundle axis, the core-to-clad diameterratios of the plural constituent optical fibers are configured todecrease as a function of radial displacement from the central bundleaxis. In still more specific alternative versions in which core-to-claddiameter ratios vary as a function of radial position relative to thecentral bundle axis, the fiber diameters of the constituent opticalfibers may be configured, in one case, to remain constant as a functionof radial displacement from the central bundle axis and, in anothercase, to vary as a function of radial displacement from the centralbundle axis.

Representative embodiments are more completely described and depicted inthe following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts a linear fused image conduit of the existing art, whichis also referred to as a “straight through;”

FIG. 1B shows a fused optical fiber image-inverting conduit of theexisting art;

FIG. 2 schematically depicts a cross-section of an optical fiber bundle,including an image-inverting optical fiber bundle, embodying aspects ofthe present invention in which fiber diameter varies as a function ofradial displacement from the central bundle axis;

FIG. 2A is a side perspective view of an image-inverting optical fiberbundle fabricated in accordance with the present invention;

FIG. 3 is a schematic cross-section an image-inverting optical fiberbundle in which fiber diameter is constant but core diameter varies as afunction of radial displacement from the central bundle axis; and

FIG. 4 is a schematic cross-sectional illustration of an alternativeimage-inverting optical fiber bundle in which fiber diameter is constantbut core diameter varies as a function of radial displacement from thecentral bundle axis.

DETAILED DESCRIPTION

The following description of variously embodied foveal image invertersand methods of fabricating the same is demonstrative in nature and isnot intended to limit the invention or its application of uses.Accordingly, the various implementations, aspects, versions andembodiments described in the summary and detailed description are in thenature of non-limiting examples falling within the scope of the appendedclaims and do not serve to define the maximum scope of the claims.

Many of the steps relative to the fabrication of a standard fusedoptical fiber bundle, including the special case of a fused-bundle imageinverter, are applicable to the fabrication of optical fiber bundles andimage inverters within the scope and contemplation of the presentinvention. These steps were summarized in the background withconjunctive reference to FIGS. 1A and 1B. Accordingly, principalreference is made to FIG. 2 for purposes of describing fundamentaldifferences between standard fused optical fiber bundles, includingstandard fused-bundle image inverters, and foveal optical fiber bundlesand image inverters within the scope and contemplation of the presentinvention as defined in the claims appended hereto. To the extent thatthe standard optical fiber bundles of FIGS. 1A and 1B include elementsin common with embodiments within the scope of the invention, they maybe referred to in order to support disclosure of the same.

Shown schematically in FIG. 2 is a cross-section of a rigidimage-conducting optical fiber bundle 10 comprising a plurality ofconstituent optical fibers 20 and extending lengthwise along a centralbundle axis A_(CB) between image-input and image-output ends 16 and 18.Each optical fiber 20 includes a core 22 and a cladding 24 extendingbetween a fiber first end 26 and a fiber second end 28 coinciding with,respectively, the image-input and image-output ends 26 and 28 of theoptical fiber bundle 10.

In various aspects, the core 22 and the cladding 24 compriseoptically-transmissive materials of differing indices of refraction,thereby facilitating internal reflection, as is known in the art. Inalternative versions, at least one of the core 22 and cladding 24comprises a glass. However, within the scope and contemplation of theinvention are embodiments in which at least one of the core 22 andcladding 24 comprises a polymeric material (e.g., a plastic). Inalternative variations in which at least one of the core 22 and cladding24 is fabricated from a polymer, the cladding 24 and core 22 of eachoptical fiber 20 may be mutually joined or “fused” by, for example, heatfusion, as is typical of glass fibers, or by an alternative means suchas an optical epoxy. Similar alternative methods may be used to joinplural optical fibers 20 within the optical fiber bundle 10 to oneanother. In still additional versions, optical fibers 20 each of whichcomprises as least one of a glass and a polymer, may be retained intheir respective positions within the overall optical fiber bundle 10 bya matrix 30 independent of the material from which the claddings 24 areformed. The matrix 30 could comprise, by way of example, a polymer,glass and/or optical epoxy.

Because a schematic cross-section is under consideration, it will bereadily understood that the optical fiber bundle 10 could be either astraight-thru or an image inverter since planar cross-sections of thesetypes of fused bundles could be indistinguishable. Moreover, while theconstituent optical fibers 20 of the optical fiber bundle 10 in FIG. 2are hexagonally packed, it is to be understood that the overallinventive concept encompasses numerous alternative fiber-packingarrangements, such as, by way of non-limiting example,circular/cylindrical, square, rectangular, etc., and that the packingarrangement is not at the precise point of novelty. A similarobservation applies to the cross-sectional geometry of the constituentoptical fibers 20 themselves. In fact, in addition tocircular/cylindrical, square, rectangular, hexagonal, etc., at least oneembodiment envisions use of five-sided fibers 20 in order to, in part,allow for the inclusion of interstitial EMA (extramural absorption)fibers among the optical fibers 20 within the bundle 10. Accordingly,and importantly, because the optical fibers 20 and bundles 10 may be ofvarious cross-sectional geometries, the use of the term “diameter” notimply a circular cross-sectional geometry. More specifically, although“diameter” is frequently thought of narrowly as the longest chord thatcan be fitted within the curve defining a circle, the more technicalmathematical definition of that term is applicable to this descriptionand the appended claims. For instance, chords within squares,rectangles, hexagons, and even, irregular shapes are also diameters.Accordingly, nothing in the preceding explanation, the detaileddescription, the appended claims or the drawings should be construed toattribute to the term “diameter” a meaning more narrow than common usageand technical mathematical usage would attribute to them. Moreover,“radius” is in all cases, unless otherwise specified, as half the lengthof a given diameter.

Representative of the inventive concept is the inclusion with theoptical fiber bundle 10 of a plurality of fiber zones Z₁ through Z_(x)concentrically arranged about the central bundle axis A_(CB), wherein Z₁is the fiber zone located closest to and/or including the central bundleaxis A_(CB) and Z_(x) is the “outermost” fiber zone (i.e, the fiber zoneradially most distant from the central bundle axis A_(CB)). Moreover,the fiber zones Z₁ through Z_(x) are populated by optical fibers 20, thefiber diameters D_(F) of which are zone dependent. More specifically,the average overall fiber diameter D_(F) of constituent optical fibers20 within the first fiber zone Z₁ is smaller than the average overallfiber diameter D_(F) of constituent optical fibers 20 within each fiberzone more radially distant from the A_(CB). More generally, the averageoverall fiber diameter D_(F) of the optical fibers 20 within each fiberzone is greater (larger) than the average overall fiber diameter D_(F)of the optical fibers 20 within each fiber zone closer to the centralbundle axis A_(CB) and less (smaller) than the average overall fiberdiameter D_(F) of the optical fibers 20 within each fiber zone moreradially distant from the central bundle axis A_(CB).

By way of concrete non-limiting example, the illustrative optical fiberbundle 10 of FIG. 2 includes four “fiber zones” referenced as Z₁, Z₂, Z₃and Z₄ (x=4 in this case). Fiber zone Z₁ (the innermost “central zone”)includes optical fibers 20 having the smallest fiber diameters D_(F)(e.g. 2.0-3.0 microns). Fiber zone Z₂ is the first zone concentricallydisposed about fiber zone Z₁ and includes optical fibers 20 ofincrementally larger fiber diameter D_(F) (e.g. 3.0-6.0 microns) thanthe optical fibers 20 of fiber zone Z₁. Concentrically disposed aboutfiber zone Z₂ is fiber zone Z₃ which is comprised of optical fibers 20of still larger fiber diameter D_(F) (e.g., 6.0-10.0 microns). Theoutermost zone, fiber zone Z₄, which is concentrically disposed aboutfiber zone Z₃, includes optical fibers 20 with the largest overall fiberdiameter D_(F) in the bundle 10 (e.g. 10.0-12.0 microns).

Referring still to the cross-sectional view of FIG. 2, and mostparticularly the enlarged view of a constituent optical fiber 20, eachoptical fiber 20, regardless of the fiber zone Z₁ through Z_(x) withwhich it is associated, has a core 22 with a core diameter D_(core) anda cladding with a cladding diameter D_(clad). Since, at least among someembodiments, it is the “outer diameter” of the cladding 24 that isrelevant, and designated as the cladding diameter D_(clad), the claddingdiameter D_(clad) may, in various aspects, be viewed as synonymous withthe fiber diameter D_(F). In any event, while the fiber diameters D_(F)themselves increase as a function of fiber zone Z and bundle radius RB,various embodiments are configured such that the core-to-clad diameterratio R_(CC), defined as D_(core)/D_(lad), is held constant across fiberzones Z₁ through Z_(x).

Another way of conceptualizing the core-to-clad diameter ratio R_(CC) isin terms of the cross-sectional area of a constituent optical fiber 20that each of the core 22 and cladding 24 of the same represents. Forexample, consider first and second optical fibers 20 with the samecladding diameter D_(clad). If the first optical fiber 20 has a corediameter D_(core) that is smaller than the core diameter D_(core) of thesecond optical fiber 20, then the first optical fiber 20 has a smallercore-to-clad diameter ratio R_(CC) than the second optical fiber 20.Expressed in alternative terms, for an optical fiber 20 of a givencladding diameter D_(clad), an increase in the core diameter D_(core)corresponds to the core 22 representing an increased percentage of theoverall cross-sectional area of the optical fiber 20 and the cladding 24representing a correspondingly decreased percentage of thecross-sectional area of the optical fiber 20.

Relative to the formation of an optical fiber bundle 10 in which theoutput image is angularly displaced relative to the input image, it willbe appreciated that an arrangement of optical fibers 20 to form a fiberbundle 10 such as that in FIGS. 2 and 2A facilitates a greater “twistrate” because the larger-diameter optical fibers 20 in the fiber zonesmore distant from the central bundle axis A_(CB), while being elongatedthe most during the twist, are not constricted down to such a smallfiber diameter D_(F) that their light-transmission capabilities arenegatively impacted. Because the “twist rate” can be much greater usingthe present solution, image-inverting optical fiber bundles 10 of muchshorter overall length are rendered possible. Moreover, the resolutionof the inverted image can be made constant across the bundle diameterD_(B) by not degrading appreciably as a function of radial displacementfrom the central bundle axis A_(CB).

In accordance with an alternative configuration, fiber diameter D_(F) isheld constant among and across fiber zones Z₁ through Z_(x), while thecore-to-clad diameter ratio R_(CC) varies as a function of fiber zone Z.In one version, an innermost first fiber zone Z₁ comprises opticalfibers 20 with a relatively high core-to-clad diameter ratio R_(CC), anoutermost third fiber zone Z₃ with a relatively low core-to-claddiameter ratio R_(CC), and at one intermediate second fiber zone Z₂situation between the innermost and outermost fiber zones Z₁ and Z₃ andcomprising optical fibers 20 with a core-to-clad diameter ratio R_(CC)between the core-to-clad diameter ratios R_(CC) of the first and thirdfiber zones Z₁ and Z₃. In some such versions, the optical fibers 20within each fiber zone Z are configured in accordance with a uniformspecification to have the same core-to-clad diameter ratio R_(CC)throughout the zone Z. However, in other variants, there is a mix withinat least one fiber zone Z of optical fibers 20 having disparate corecore-to-clad diameter ratios R_(CC). Examples of two alternativeconfigurations of bundles 10 having variable core core-to-clad diameterratios R_(CC) across fiber zones Z are discussed in further detail belowwith conjunctive reference to the schematically-represented bundlecross-sections shown in FIGS. 3 and 4.

Referring now to FIG. 3, there is shown a cross-section of a rigidimage-conducting optical fiber bundle 10 comprising a plurality ofconstituent optical fibers 20 generally situated in first, second, andthird fiber zones Z₁, Z₂, and Z₃. Within each fiber zone Z₁, Z₂, and Z₃the core-to-clad diameter ratios R_(CC) are constant, but they aredisparate among the fiber zones Z. More specifically, within the first,second, and third fiber zones Z₁, Z₂, and Z₃, the core-to-clad diameterratios R_(CC) are, respectively, R_(CC1), R_(CC2), and R_(CC3). In thisparticular case, the fiber diameters D_(F) are constant across theentire bundle 10. That is, the cladding diameters D_(clad) are constantamong the first, second, and third fiber zones Z₁, Z₂, and Z₃ so thatthe variance in the core-to-clad diameter ratios R_(CC1), R_(CC2), andR_(CC3) is attributable to variance among the core diameters D_(core)across the first, second, and third fiber zones Z₁, Z₂, and Z₃. Morespecifically, as generally indicated in FIG. 3, although not preciselytied to any particular scale, the cores 22 of the optical fibers 20 inthe first fiber zone Z₁ are configured with the largest core diameterD_(core), the cores 22 of the optical fibers 20 in the second fiber zoneZ₂ have the second-largest core diameter D_(core), and cores 22 of theoptical fibers 20 in the third fiber zone Z₃ have the smallest corediameter D_(core). Given the constant cladding diameter D_(clad) acrossall fiber zones Z, it follows that R_(CC1)>R_(CC2)>R_(CC3) and indicatedin FIG. 3.

With reference to FIG. 4, another variation of an embodiment withvarious core-to-clad diameter ratios R_(CC) is briefly described.Although numerous permutations within the overall scope andcontemplation of the invention are possible, a broader overall conceptis represented more simplistically through a discussion of FIG. 4. Likethe illustrative example of FIG. 3, the example of FIG. 4 generallyincludes first, second, and third fiber zones Z₁, Z₂, and Z₃. Moreover,as with the example of FIG. 3, and in order to keep the examplemanageable and simple, the cladding diameters D_(clad) are constantamong the first, second, and third fiber zones Z₁, Z₂, and Z₃ so thatany variance in the core-to-clad diameter ratios R_(CC) among opticalfibers 20 is attributable to variance among the core diameters D_(core).

A key difference between the examples of FIGS. 4 and 3 is that, in theexample of FIG. 4, there is, by design, a variance in the core-to-claddiameter ratio R_(CC) within at least an intermediate zone (e.g., fiberzone Z₂ in this case). While there may be some variance in thecore-to-clad diameter ratio R_(CC) within each of the fiber zones Z, oracross some radially-dependent gradient, for simplicity, a variancewithin only fiber zone Z₂ is considered. While the first fiber zone Z₁includes only optical fibers 20 with a first core-to-clad diameter ratioR_(CC1) and the third fiber zone Z₃ includes only optical fibers 20 witha third core-to-clad diameter ratio R_(CC3) lesser in magnitude than thefirst core-to-clad diameter ratio R_(CC1), the second fiber zone Z₂includes a mixture of core-to-clad diameter ratios R_(CC). In thisparticular case, as illustrated by the three demonstrative fibercross-sections just above the bracket labeled “Variable R_(CC) in Z₂,”there are within fiber zone Z₂ optical fibers 20 characterized by thefirst, second, and third core-to-clad diameter ratios R_(CC1), R_(CC2),and R_(CC3). Whereas the second core-to-clad diameter ratio R_(CC2) isunique to the second fiber zone Z₂ in this configuration, as previouslymentioned, the first fiber zone Z₁ is populated entirely by opticalfibers 20 of the first core-to-clad diameter ratio R_(CC1), while thethird fiber zone Z₃ is populated entirely by optical fibers 20 of thethird core-to-clad diameter ratio R_(CC3).

Whether or not the second fiber zone Z₂ of any particular embodimentincludes fibers 20 of a second core-to-clad diameter ratio R_(CC2)unique to the second fiber zone Z₂, the inclusion within the secondfiber zone Z₂ of a mixture of optical fibers 20 exhibiting both thefirst and third core-to-clad diameter ratio R_(CC1) and R_(CC3) resultsin a smoother, less obvious transition between fibers 20 of the firstand third fiber zones Z₁ and Z₃. Optical fibers 20 of disparatecore-to-clad diameter ratio R_(CC) may be randomly distributed in thesecond fiber zone Z₂ or well ordered. In at least one configuration, theinclusion within the second fiber zone Z₂ of optical fibers 20 of thefirst core-to-clad diameter ratio R_(CC1) is more dense nearer the firstfiber zone Z₁, while the inclusion in the second fiber zone Z₂ ofoptical fibers 20 of the third core-to-clad diameter ratio R_(CC3) ismore dense nearer the third fiber zone Z₃ thereby defining a sort ofradially-dependent inclusion gradient of disparate core-to-clad diameterratios R_(CC) within the bundle 10. In addition to avoiding visible“steps” between fiber zones Z, the inclusion of optical fibers 20 ofdisparate core-to-clad diameter ratio R_(CC) in different concentrationsacross the cross-section of the bundle 10 in accordance with a radialgradient mitigates structural stresses associated with the heating,drawings and twisting steps, and with the differing effects of thermalexpansion on disparate fiber types.

Because variances in core-to-clad diameter ratios R_(CC) even withineach fiber zone Z are envisioned, it may be useful to conceptualize thecore-to-clad diameter ratio R_(CC) within any particular fiber zone Z asrepresentative of an average core-to-clad diameter ratio R_(CC)applicable to that zone, and the averages among all fiber zones Z asdecreasing with radial displacement from the central bundle axis A_(CB).In accordance with this conceptualization, FIG. 4 indicates that“average R_(CC1)>average R_(CC2)>average R_(CC3). In a manner similar tointegration in calculus, as the number of annular fiber zones Z becomesvery large, and the annular thickness of each fiber zone Z becomes verysmall, the more arbitrary the designation of individual fiber zones Zbecomes, and the radially dependent gradient fiber inclusion becomesincreasingly “smooth.”

Beyond the preceding, it is to be understood that, within any givenconfiguration, variances in overall fiber diameter D_(F) andcore-to-clad diameter ratios R_(CC) are not mutually exclusive. Morespecifically, while the description up to the present has consideredalternative configurations in which, on the one hand, fiber diameterD_(F) varies as a function of radial displacement from the centralbundle axis A_(CB) while core-to-clad diameter ratio R_(CC) is constantacross the bundle 10 and, on the other hand, fiber diameter D_(F) isconstant across the bundle 10 while core-to-clad diameter ratio R_(CC)varies as a function of radial displacement from the central bundle axisA_(CB), expressly within the scope and contemplation of the inventionare bundle configurations in which both fiber diameter D_(F) andcore-to-clad diameter ratio R_(CC) varies as a function of radialdisplacement from the central bundle axis A_(CB).

The foregoing is considered to be illustrative of the principles of theinvention. Furthermore, since modifications and changes to variousaspects and implementations will occur to those skilled in the artwithout departing from the scope and spirit of the invention, it is tobe understood that the foregoing does not limit the invention asexpressed in the appended claims to the exact constructions,implementations and versions shown and described.

What is claimed is:
 1. A rigid image-conducting optical fiber bundleextending longitudinally along a central bundle axis between an imageinput end and an image output end and being twisted about the centralbundle axis along a portion of its length such that an image inputtedinto the image input end is angularly displaced about the central bundleaxis before being outputted through the image output end, the bundlecomprising: a plurality of adjacently fused constituent optical fibers,each optical fiber having a first end coinciding with the image inputend and a second end coinciding with the image output end, wherein, asviewed into a selected cross-section of the optical fiber bundle (i)each constituent optical fiber includes a cladding with a claddingdiameter corresponding with the fiber diameter of that optical fiber anda core with a core diameter, and the ratio of the core diameter to thecladding diameter defines a core-to-clad diameter ratio relative to thatoptical fiber; (ii) the bundle includes a plurality of at least twofiber zones including a at least a first fiber zone and a second fiberzone, the first and second fiber zones being concentrically arrangedabout the central bundle axis and defined such that the first fiber zoneis nearer the central bundle axis than is the second fiber zone; (iii)each fiber zone is populated by a plurality of optical fibers configuredin accordance with a uniform specification relative to at least one offiber diameter and core-to-clad ratio; and (iv) among the fiber zones,at least one of (a) the zone dependent fiber diameters of theconstituent optical fibers increase as a function of radial displacementfrom the central bundle axis; and (b) the zone dependent core-to-claddiameter ratios of the constituent optical fibers decrease as a functionof radial displacement from the central bundle axis.
 2. The opticalfiber bundle of claim 1 wherein, as viewed into the cross-section, thecore-to-clad diameter ratios of the plural constituent optical fibersare configured to decrease as a function of radial displacement from thecentral bundle axis.
 3. The optical fiber bundle of claim 2 wherein, asviewed into the cross-section, the fiber diameters of the constituentoptical fibers are configured to remain constant as a function of radialdisplacement from the central bundle axis.
 4. The optical fiber bundleof claim 1 wherein, as viewed into the cross-section, the fiberdiameters of the constituent optical fibers are configured to remainconstant as a function of radial displacement from the central bundleaxis.
 5. The optical fiber bundle of claim 4 wherein at least one of thecore and the cladding comprises a polymeric material.
 6. The opticalfiber bundle of claim 1 wherein at least one of the core and thecladding comprises a polymeric material.